A cysteine residue comprised in a peptide may be oxidized in a solution to produce a disulfide bond. A peptide comprising a reduced cysteine residue and a peptide comprising an oxidized one are drastically different each other in the structure, and, in the course of the use of them as cancer vaccines, CTLs specific for one of the peptides are not always reactive to the other (Immunity 1997; 6: 273-281). Thus, when a cancer antigen peptide comprising a cysteine residue is designed as a pharmaceutical composition of a cancer vaccine, it would be believed to provide an advantage that a peptide wherein a cysteine residue comprised in the cancer antigen peptide is substituted with another amino acid residue is developed in stead of the antigen peptide. However, a peptide wherein a cysteine residue is substituted with another amino acid residue does not necessarily function as a cancer antigen peptide, and such substituted type peptides provide largely different efficacy (J. Immunol., 1998; 161:6985-6992, J. Immunol., 1998; 160:2099-2106).
WT1235-243 (Cys-Met-Thr-Trp-Asn-Gln-Met-Asn-Leu; SEQ ID NO: 2), that is a peptide spanning in positions 235 to 243 of the cancer antigen protein, WT1 (SEQ ID NO: 1, Cell., 60:509, 1990), is a cancer antigen peptide having an activity to induce CTLs in HLA-A24-restricted manner (Clin. Cancer. Res. 8: 2626, 2002, and WO 00/06602). The altered peptide (Cys-Tyr-Thr-Trp-Asn-Gln-Met-Asn-Leu; SEQ ID NO: 3, hereinafter it may be referred to as WT1235-243 (2M→Y)) wherein the methionine at position 2 of WT1235-243 is altered to tyrosine has a higher binding activity to the HLA-A24 antigen than the natural type peptide (WO 02/079253, the international publication date: Oct. 10, 2002). The natural type peptide, WT1235-243, and the altered peptide, WT1235-243 (2M→Y), are both expected to become an active agent for cancer immunotherapy.